I agree. The restriction is justified by the prohibitions outlined in the customs of druidry (as defined by D&D), but druids won't go to hell for rocking some dope elven chainmail. Heh
But that could in fact be the reason.
(At a party 25,000 years ago)
Druid: "Finally my time has come. I've been invited to the annual Gods' Bash. What should I wear? I know! I got this new set of shiny magical plate mail. That should make an impression."
(A few days later) said in Sponge Bob narrator voice.
Druid: "I'm so happy! I've met Torm, Sune, Apollo and Thor. I can't wait to see who I run into next."
A drunk Dionysus:
hic "Hi. Who are you and what is that you're wearing?"
Druid: "My name is Thistle Thornbush and this is my new armor. Do you like it?"
A drunk Dionysus: "It's, um, very nice, but you do know that if a druid wears metal armor he'll be taken to Hell and tortured forever when he dies, right?"
Druid: (a look of horror comes across the druid's face) "Excuse me! I have to go!"
(later that night)
A drunk Dionysus: "Hey Demeter, you'll never guess what I told one of your druids.
snicker I told him that druids who wear metal armor go to Hell."
(A drunk Dionysus and Demeter fall to the ground laughing hysterically.)
And that ladies in gentlemen, is why druids won't wear metal armor.